Standing wave linear particle beam accelerators are characterized by plural cascaded standing wave electromagnetically coupled main cavities having approximately the same resonant frequency and plural side cavities. Adjacent ones of the main cavities are electromagnetically coupled to a common side cavity. A beam of charged particles, usually electrons, is injected into the main cavities so the beam travels longitudinally through the cascaded cavities. The cavities are excited with an electromagnetic wave having a frequency that is approximately equal to the resonant frequency of the main cavities so that there is normally a fixed phase shift of 180 degrees between adjacent main cavities.
Such standing wave linear accelerators are widely used for medical, radiation therapy and industrial, radiographic applications. One class of such devices operates in the energy range from 2-5 million electron volts (MeV). To provide for the complete energy range from 2 to 5 MeV, the voltage of the RF applied to the standing wave structure must be changed. However, changing the voltage of the injected microwave energy concommitantly changes the diameter of the particle beam applied to the treated area. It is usually desirable, however, to control the diameter of the particle beam applied to the treated area so that the diameter remains constant for differing energy levels. In other instances, it is desirable to vary the diameter of the output beam irradiating the treated subject matter when there is no change in the beam energy.